This invention relates to an injection blow molding method for producing double layered hollow articles. More particularly, the invention relates to the injection blow molding of double layered hollow articles which are made of two layers of thermoplastic synthetic resins of different kinds or different colors.
In the prior art method, the double parison technique has been employed for obtaining double layered hollow articles in which double layered parison is placed in a blow mold cavity and air is blown into said parison. In order to form the double layered parison, the molten resin is injected molded on the outside of inner parison to form thereon an outer parison. In this stage, however, the outside of said inner parison is fused by the heat and pressure of the injection of the outer parison, and sometimes, the inner parison and outer parison are fused and mixed together to lose the definition between the double layered parison. Especially, the portion of inner parison opposite to the gate is liable to be fused and deformed by the injection pressure, so that the thickness distribution of the inner parison becomes uneven and the thickness of the product through the blow molding process becomes also uneven. Further, in the blow molding process, it is rather difficult to properly blow the double layered parison in like manner as a single layered parison, and the thicker the parison is, the more difficult it becomes to form a hollow article with uniform thickness. And with such uneven thickness product, the characteristic advantages of the double layered structure can not be expected.